1. Technical Field of the Invention
The present invention relates to a turbo compressor, and more particularly to a turbo compressor in which a service life of bearings is elongated and a critical speed of a rotating shaft is improved.
2. Description of Related Art
In a refrigerating machine, there is employed a centrifugal compressor, so-called turbo compressor, for compressing a coolant gas serving as a working fluid to bring the compressor in the high temperature and high pressure state.
Meanwhile, in the compressor, if a compression ratio is higher, a discharge temperature of the compressor becomes higher and a volumetric efficiency is lowered. Particularly, if the evaporation temperature becomes lower, the compression ratio becomes higher, and accordingly, there is a case that a compressing operation is divided into two stages, three stages or more stages. The turbo compressor in which the compressing operation is executed by multiple stages in this manner is called as a multistage turbo compressor.
As a prior art of a two-stage turbo compressor, there is one disclosed in the following patent document 1, and the structure thereof is shown in FIG. 1.
In this turbo compressor 80, a first stage centrifugal impeller 83 and a second stage centrifugal impeller 84 are fixed to a rotating shaft 82, which is rotatably provided in a housing 81, such that the first stage centrifugal impeller 83 and the second stage centrifugal impeller 84 are arranged at an interval therebetween and in the same orientation.
The rotating shaft 82 is rotatably supported at axially spaced apart positions thereof by a bearing A and a bearing B, in such a state that a portion of the rotating shaft 82 to which the first stage centrifugal impeller 83 and the second stage centrifugal impeller 84 are fixed overhangs.
The bearing A is constituted by a combined angular ball bearing using angular ball bearings, and the bearing B is constituted by a combined angular ball bearing using two angular ball bearings.
Further, an output shaft 86 of a motor 85 serving as a drive source is rotatably supported by a bearing 87. A large gear 88 is fixed to the output shaft 86, and a small gear 89 engaging with the large gear 88 is fixed to the rotating shaft 82, whereby the rotational force of the output shaft 86 of the motor 85 is transmitted to the rotating shaft 82, with the increased speed.
In the turbo compressor 80 structured as mentioned above, the coolant is compressed by the first stage centrifugal impeller 83 on the upstream side, then introduced into the second stage centrifugal impeller 84 to be further compressed, and then delivered to the outside.
Further, there is disclosed in the following patent document 2 a structure in which impellers are fixed to opposite end portions of a rotating shaft of a turbo compressor, an output shaft of a motor is coupled to a center portion of the rotating shaft, and bearings are arranged near the opposite end portions of the rotating shaft.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-303298
Patent Document 2: Japanese Laid-Open Patent Publication No. 5-223090
In the compressor, the pressure on the back side of the impeller is higher than the pressure on the front side of the impeller. This pressure difference generates a thrust force in the impeller from the back side toward the inlet side. Accordingly, if two impellers are arranged in the same orientation such as those in the turbo compressor in the patent document 1, the thrust forces applied to both the impellers are combined to generate a great thrust force. Therefore, as to the bearing which supports a larger thrust load applied to the rotating shaft of the compressor, a mechanical loss becomes larger as the support load becomes larger, and there is a problem that a service life of the bearing becomes short. Further, if the number of the bearings arranged is increased in order to elongate the service life of the bearings, there is a problem that the mechanical loss becomes large.
Further, in the turbo compressor in the patent document 2, the angular ball bearing is employed as the bearing. The angular ball bearing can receive not only a radial load but also a thrust load, however, in order to receive the thrust load in opposite directions, it is necessary to use two or more angular ball bearings in combination. Accordingly, the number of the bearings to be used is increased, and there is a problem that the mechanical loss is large.
Further, in the structure in which a plurality of impellers are attached to the overhang portion of the rotating shaft, such as in the turbo compressor in the patent document 1, it is necessary to take a step such as a step for shortening the axial length of the impellers in the case of taking the critical speed of the rotating shaft into consideration.
However, it is not preferable in the light of the compression efficiency to shorten the axial length of the impeller.
Further, in the turbo compressor in the patent document 2, the distance between the shaft support portions is elongated because of being supported near opposite end portions of the rotating shaft, causing a problem that the critical speed is lowered.